Optical discs as low-cost, quasi-random nanoimprinting templates for photon management

ABSTRACT

Photonic devices are provided comprising a photoactive layer and at least one additional layer, wherein a surface of the photoactive layer or a surface of the at least one additional layer has imprinted thereon a quasi-random pattern of nanostructures corresponding to a quasi-random pattern of nanostructures defined in a recording layer of a pre-written optical media disc. Methods of patterning a layer of a photonic device are also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.14/839,112 filed on Aug. 28, 2015, the entire contents of which arehereby incorporated by reference; which claims priority from U.S.Provisional Patent Application No. 62/043,696 filed on Aug. 29, 2014,the entire contents of which are hereby incorporated by reference.

REFERENCE TO GOVERNMENT RIGHTS

This invention was made with government support under grant numbersCMMI1130407 and CMMI1130640 awarded by the National Science Foundation.The government has certain rights in the invention.

BACKGROUND

Disrupting the ordering of periodic structures results in so-calledquasi-random photonic structures, which can provide significantflexibility when engineering the optical response of materials¹⁻⁴.Random and quasi-random photonic structures are abundant in many speciesin nature⁵⁻⁷ and have been adopted for diverse engineering applications,such as highly-efficient photon extraction in light-emitting diodes⁸,biomimetic structural coloration^(5,9), and random lasing^(10,11).Quasi-random nanostructures are advantageous for light trapping in thinfilm solar cells because, unlike perfectly periodic¹²⁻¹⁶ or totallyrandom structures, they can offer both broadband absorption enhancementand customizable spectral response for different photoactivematerials^(4,17-19). Subwavelength disordered nanostructures aretypically custom-made via advanced lithography over a limited area.

SUMMARY

Provided are methods for patterning a layer of a photonic device usingstamps derived from optical media discs. The stamps, the photonicdevices comprising the patterned layer, and related methods are alsoprovided.

In one aspect, a method of patterning a layer of a photonic device isprovided, the method comprising pressing a stamp on a surface of a layerof a photonic device, the stamp comprising a stamping surface whichdefines a negative replica of a quasi-random pattern of nanostructuresdefined in a recording layer of a pre-written optical media disc, for aperiod of time sufficient to imprint the quasi-random pattern ofnanostructures defined in the recording layer of the pre-written opticalmedia disc onto the surface of the layer of the photonic device; andremoving the stamp.

In another aspect, a photonic device is provided, the photonic devicecomprising a photoactive layer and at least one additional layer,wherein a surface of the photoactive layer or the at least oneadditional layer has a quasi-random pattern of nanostructures definedtherein, the quasi-random pattern of nanostructures substantiallymatching a quasi-random pattern of nanostructures defined in a recordinglayer of a pre-written optical media disc.

Methods of patterning a layer of a photonic device usingphotolithographic masks, the photolithographic masks, and the photonicdevices formed using the photolithographic masks are also provided.

Other principal features and advantages of the invention will becomeapparent to those skilled in the art upon review of the followingdrawings, the detailed description, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments of the invention will hereafter be describedwith reference to the accompanying drawings.

FIGS. 1A-1D depict the Fourier transformations of subwavelength featuresarranged in periodic, random, quasi-random and Blu-ray patterns. Imageson the left are real space images and images on the right areFourier-space images. Scale bar, 2.5 μm. See FIG. 1D for axes labels.The circles in the images on the right denote the largest (outercircles) and smallest (inner circles) k-values needed to couple theentire solar spectrum (315 nm to 2.5 μm) to surface plasmons at theinterface between an example photoactive layer, PTB7:PC-₇₁BM, and silverelectrode. FIGS. 1A (a periodic pattern) and 1B (a random pattern) yieldwavevectors that either too discretized or too diffuse to lie betweenthe two circles, in contrast to FIG. 1C, which shows an optimizedquasi-random pattern. FIG. 1D shows a Blu-ray pattern, obtained bythresholding an AFM image of the recording layer of a movie disc (seeFIGS. 3A-3C), produces close-to-optimized distribution of k-values.

FIGS. 2A-2C depict the processing of Blu-ray patterned solar cells. FIG.2A shows a schematic diagram of the process for: (top left) delaminatinga Blu-ray Disc and casting a PDMS mold on the exposed recording layer;(bottom left) fabricating the nanopatterned solar cells; and (middleright) imprinting the active layer using the nanopatterned PDMS mold andevaporating the MoO₃/Ag electrode. FIG. 2B shows an AFM image of thenanopatterned PDMS mold. FIG. 2C shows a schematic diagram of the solarcell architecture used in the numerical and experimental portions ofExample 1.

FIGS. 3A-3C show the physical morphology and performance characteristicsof Blu-ray patterned solar cells. FIG. 3A shows AFM images of the activelayer without stamping (top left) and after stamping with a Blu-raypatterned mold (top right). The horizontal line profiles (bottom)illustrate the depth contrast of the Blu-ray patterned active layer whencompared to the non-patterned device. FIG. 3B shows optical images of ahalf-patterned solar cell showing iridescent scattering due to theperiodic nature of the Blu-ray pattern (top) and non-iridescent,broadband scattering from the non-patterned half (bottom). FIG. 3C showsreflected absorption (1-R) measurements from typical Blu-ray patternedand non-patterned solar cells. FIG. 3D shows that measured externalquantum efficiencies (EQE) show a broadband enhancement and agree wellwith the calculated absorption spectrum in FIG. 3C.

FIGS. 4A-4C show the Blu-ray Disc format and structure. FIG. 4A shows anAFM image and FIG. 4B shows a SEM image of a delaminated, read-onlyBlu-ray movie disc showing the superimposed periodicity (x-axis) andrandomness (y-axis) of the pits. Scale bar, 500 nm. FIG. 4C shows aschematic illustrating the cross-section structure of a read-onlyBlu-ray disc and the corresponding optical read-out system.

FIGS. 5A-5C show the feature sizes of optical media discs. The AFMimages and Fourier transformation are shown for Blu-ray (FIG. 5A), DVD(FIG. 5B), and CD (FIG. 5C) discs. See FIG. 5C for axes labels. TheFourier transformation was conducted on 10 μm cell size corresponding tothe AFM images. The outer and inner circles represent the largest andsmallest wave vectors needed to couple visible spectrum (315 nm to 2 μm)to surface plasmon existing at the interface between PTB7:PC₇₁BM andsilver.

FIG. 6 shows the Fourier distributions of data from two separate areasof a Blu-ray disc. Fourier distributions of data from several otherareas of the Blu-ray disc were also obtained. The Fourier transform ofmultiple AFM images of Blu-ray patterns (left images) all yieldapproximately the same distribution in Fourier-space.

FIGS. 7A-7E show simulated absorption spectra of an example polymersolar cell imprinted with the Blu-ray pattern. FIG. 7A shows a schematicof the thin film polymer solar cell used in the 1-dimensionalsimulation. FIG. 7B shows an AFM image of the recording layer of aBlu-ray disc. The four lines were used as a representative example ofthe quasi-random structure. FIGS. 7C and 7D show the absorption spectraunder TE and TM polarizations, respectively. The blue curves (bottommostcurves) represent the absorption of a non-patterned solar cell with thesame volume of active material. FIG. 7E shows that an average of thesimulated enhancement curves displays a broadband enhancement.

FIG. 8 shows solar cell characterization. Characteristic current-densityvs. voltage plots for non-stamped and Blu-ray stamped PTB7:PC₇₁BM solarcells with an active layer thickness of 50 nm are shown.

FIGS. 9A-9C show an assessment of Blu-ray pattern for other solar cellmaterials. Each box shows the Fourier transform of the Blu-ray patternat the interfaces of active layers of different photoactive materials(PTB7:PC₇₀BM (FIG. 9A), P3HT:PCBM (FIG. 9A); amorphous silicon (FIG.9B); perovskite (FIG. 9B); CdTe (FIG. 9C); GaAs (FIG. 9C)) andelectrodes of different materials (Al; Ag; Au). See FIG. 9C for axeslabels. The circles denote the largest (outer) and smallest (inner)k-values needed to couple the entire solar spectrum (315 nm to 2.5 μm)to surface plasmons at the interface between the various photoactive andelectrode materials.

FIGS. 10A-10C compare the performance of devices stamped with a flat-vs. a Blu-ray-patterned mold. FIG. 10A shows atomic force microscopyimages of the active layer after stamping with a flat stamp (left) andwithout stamping (right). The horizontal line profiles in FIG. 10A areplotted in FIG. 10B, showing little difference between the flat- andnon-patterned active layers. FIG. 10C shows characteristiccurrent-density vs. voltage plots for flat-stamped and Blu-ray stampedPTB7:PC₇₁BM solar cells.

FIGS. 11A-11I demonstrate optical disc nanopattern manipulation. FIG.11A shows a typical compression and error-control algorithm for opticalmedia discs, in which raw media data are compressed to reduce the filesize and minimize data redundancy. Before writing to the disc the dataare controlled for sequences that could result in read-errors, narrowingthe allowable feature size range when written to the disc. FIG. 11B(top) shows a simulated projection image of a rewritable/blank opticaldisc, where white pixels represent the raised tracks and black pixelsrepresent lowered rows. FIG. 11B (bottom) shows a map which representsthe Fourier transform (FT) of the top simulated projection image,showing the first- and second-order diffraction peaks due to theperiodic nature of the tracks. As shown in FIG. 11C (top), a pre-writtenoptical media disc has distinct pits and lands, while its FT (bottom)shows pronounced scatter due to the quasi-random nature of the data. Byoverlaying multiple patterns (as shown in FIG. 11D), it is possible toreduce the polarization-dependence of the FT, as shown in FIG. 11E.Tuning the sizes of the features written to the disc (FIG. 11F) narrowsthe Fourier response, allowing for the creation of a discrete ring whenapplying the overlay technique from FIG. 11D, as shown in FIG. 11G. Bytuning the track spacing (FIG. 11H), the first order diffraction peakscan be shifted, smeared out, or even eliminated. As shown in FIG. 11I,by increasing the track spacing to equal the mean length of the featuresizes, it is possible to overlay the ring with the first orderdiffraction peaks. The scale bars next to FIG. 11B show the lengthscales corresponding to the three most commonly used types of opticaldiscs.

FIG. 12 illustrates how multiple parametric equations may be used towrite overlapping data onto an optical media disc in order to providestacked optical data storage formats such as those shown in FIGS. 11Gand 11I.

DETAILED DESCRIPTION

Provided are methods of patterning a layer of a photonic device usingstamps derived from optical media discs. The stamps, the photonicdevices comprising the patterned layer, and related methods are alsoprovided.

In a basic embodiment, a method of patterning a layer of a photonicdevice comprises pressing a stamp on a surface of a layer of a photonicdevice, the stamp comprising a stamping surface which defines a negativereplica of a quasi-random pattern of nanostructures defined in arecording layer of a pre-written optical media disc. The stamp ispressed for a period of time sufficient to imprint the quasi-randompattern of nanostructures defined in the recording layer of thepre-written optical media disc onto the surface of the layer of thephotonic device. The method further comprises removing the stamp. Byusing this method, the surface of the layer of the photonic device has aquasi-random pattern of nanostructures defined therein whichsubstantially matches the quasi-random pattern of nanostructures definedin the recording layer of the pre-written optical media disc.

The method makes use of stamps derived from pre-written optical mediadiscs in order to imprint the patterns contained within such pre-writtenoptical media discs onto layer(s) of photonic devices. Since pre-writtenoptical media discs are mass produced at extremely low cost, the presentmethods offer a significantly lower cost, higher throughput and scalablealternative to patterning layers using molds made via advancedlithographic techniques (e.g., photolithography, electron beamlithography, focused ion beams, etc.) Moreover, since the quasi-randompatterns defined in the recording layer of an optical media disc may bemanipulated by adjusting the algorithms for writing data to the opticalmedia disc, the present methods offer an efficient and cost-effectiveway to tune the quasi-random patterns to achieve a desired effect for aparticular application (e.g., maximizing light trapping over a range ofwavelengths, e.g., in order to enhance the efficiency of a photovoltaiccell).

Optical media discs are electronic storage media that can be written toand read from using a laser beam. Optical media discs are capable ofencoding data in the form of nanometer-sized pits and lands formed onthe surface of the recording layers of the optical media discs. Thestamps used in the present methods are derived from pre-written opticalmedia discs, by which it is meant optical media discs which have beenwritten to such that the recording layers have been encoded with certaindata (e.g., audio and visual data from a movie). Pre-written opticalmedia discs may be referred to as “read-only” optical media discs.Pre-written optical media discs are distinguished from blank,rewritable, or write-once optical media discs.

Pre-written optical media discs comprise a recording layer, a surface ofwhich defines a quasi-random pattern of nanostructures, thenanostructures comprising a plurality of pits and lands, generallyarranged as a continuous spiral track extending from an innermost tracknear the center of the disc to an outermost track near the outer edge ofthe disc. A pre-written optical media disc may be characterized by itsoptical data storage format which further specifies a particularconfiguration for the plurality of pits and lands making up thequasi-random pattern of nanostructures. By way of illustration, theconfiguration, and thus the optical data storage format may becharacterized by a pit width of the pits; a minimum pit length of thepits; and a track pitch. The pit width is the dimension across a pitalong the read direction (y) of the optical media disc. The pit lengthis the dimension across a pit along the radial direction (x) of theoptical media disc. The track pitch is the dimension from the center ofa track of pits/lands to the center of an adjacent track of pits/lands.

The optical data storage format of the pre-written optical media discmay be a standard format characterized a particular pit width, aparticular minimum pit length, and a particular track pitch. By way ofillustration, the Table below lists four standard optical data storageformats.

TABLE Standard Optical Data Storage Formats. Optical Data Storage FormatPit width Minimum pit length Track Pitch Compact Disc (CD) 600 nm 800 nm1.6 μm Digital Versatile Disc 320 nm 400 nm 740 nm (DVD) High Density(HD) 200 nm 200 nm 400 nm DVD Blu-Ray Disc (BRD) 130 nm 150 nm 320 nm

An AFM image of a section of the recording layer of a pre-writtenBlu-ray optical media disc is shown in FIG. 4A. A cross-sectional viewof a schematic of the pre-written Blu-ray optical media disc is shown inFIG. 4C. These figures show that the surface of the recording layer ofthe optical media disc has defined therein a quasi-random pattern ofnanostructures, the nanostructures comprising a plurality of pits andlands arranged in a plurality of tracks. The tracks of the plurality oftracks are aligned along their longitudinal axes. The quasi-randompattern of nanostructures is configured according to the Blu-ray opticaldata storage format (i.e., an optical data storage format characterizedby a pit width of 130 nm, a minimum pit length of 150 nm, and a trackpitch of 320 nm).

The optical data storage format of the pre-written optical media discmay be a non-standard optical data storage format specifying particularpit dimensions, land dimensions and/or track pitches which deviate fromthe values characterizing a standard optical data storage format. Suchdeviations may be used to tune the Fourier response of the quasi-randompattern of nanostructures to achieve a particular effect on the lightmanipulated by the patterned layer. By way of illustration, anon-standard optical data storage format may be obtained by restrictingthe length of a pit and/or the length of a land to be within aparticular range of values. As another illustration, a non-standardoptical data storage format may be obtained by altering (i.e.,increasing or decreasing or eliminating) the track pitch. As anotherillustration, a non-standard optical data storage format may be obtainedby modulating the track pitch, i.e., allowing the track pitch to bewithin a particular range of values, thereby introducing randomness intothe track pitch. Combinations of these modifications may be used toprovide the non-standard optical data storage format.

Illustrative non-standard optical data storage formats are described inExample 2, below. In particular, the quasi-random patterns ofnanostructures schematically illustrated in FIGS. 11F and 11H includethose configured according to a non-standard optical data storageformat. The quasi-random patterns themselves are shown in the top images(real space images) and the Fourier response of the quasi-randompatterns are shown in the bottom images (Fourier space images). FIG. 11Fdemonstrates the effect of the contiguous length of pits and lands onthe Fourier transform of the quasi-random pattern. In the left-mostimage of FIG. 11F, 2 or 3 contiguous bits forming pits and lands isshown; in the adjacent image, 4 or 5 contiguous bits forming pits andlands is shown; in the next adjacent image 6 or 7 contiguous bitsforming pits and lands is shown; in the right-most image 8, 9, 10 . . .20 contiguous bits forming pits and lands is shown. FIG. 11F is furtherdiscussed in Example 2, below.

FIG. 11H demonstrates the effect of track spacing on the diffractionpeaks of the Fourier transform of the quasi-random pattern. Each of theimages of FIG. 11H makes use of 6 or 7 contiguous bits forming pits andlands. In the left-most image of FIG. 11H, the track spacing (i.e.,pitch) is that dictated by a standard optical data storage format; inthe adjacent image, the track spacing is increased by a factor of threeas compared to the standard optical data storage format; in the next,adjacent image, the track spacing is allowed to vary randomly betweenfrom 0 to three times greater than that of the standard optical datastorage format; in the right-most image, the track spacing is eliminated(i.e., there is no spacing between tracks). FIG. 11H is furtherdiscussed in Example 2, below. Non-standard optical data storage formatsmay be obtained by modifying the algorithms (which already account for aparticular pit/land dimension and track pitch) used to write to opticalmedia discs.

The optical data storage format of the pre-written optical media discmay be a stacked optical data storage format in which standard opticaldata storage formats, non-standard optical data storage formats, orcombinations thereof, are stacked or overlaid. As described above, eachstandard and/or non-standard optical data storage format within thestack further specifies a particular configuration for the plurality ofpits and lands making up the quasi-random pattern of nanostructures.Thus, a quasi-random pattern of nanostructures configured according to astacked optical data storage format will be a stacked pattern comprisinga quasi-random pattern of nanostructures overlaid with one or moreadditional quasi-random patterns of nanostructures.

By way of illustration, the top image of FIG. 11E represents a stackedquasi-random pattern of nanostructures configured according to a stackedoptical data storage format (the bottom image is the correspondingFourier response). The stacked quasi-random pattern of nanostructuresincludes a first quasi-random pattern of nanostructures configuredaccording to a standard Blu-ray optical data storage format (left imageof FIG. 11D) overlaid with a second quasi-random pattern ofnanostructures configured according to a Blu-ray optical data storageformat in which the tracks have been oriented 60° relative to thestandard Blu-ray optical data storage format (middle image of FIG. 11D)overlaid with a third quasi-random pattern of nanostructures configuredaccording to a Blu-ray optical data storage format in which the trackshave been oriented 120° relative to the standard Blu-ray optical datastorage format (right image of FIG. 11D). Each quasi-random pattern ofnanostructures comprises a plurality of pits and lands arranged in aplurality of tracks, the tracks aligned along their longitudinal axes.The angle θ₁ defined by the longitudinal axes of the tracks in thesecond quasi-random pattern of nanostructures relative to thelongitudinal axes of the tracks in the first quasi-random pattern ofnanostructures is about 60°. The angle θ₂ defined by the longitudinalaxes of the tracks in the third quasi-random pattern of nanostructuresrelative to the longitudinal axes of the tracks in the firstquasi-random pattern of nanostructures is about 120°. However, thesevalues for the angles θ₁ and θ₂ are not limiting. Other angles may beused, e.g., angles in the range of greater than 0° to less than 360°,e.g., angles of 5°, 10°, 20°, 30°, etc. Similarly the number of overlaidquasi-random pattern of nanostructures is not limiting.

The top image of FIG. 11G also illustrates a stacked quasi-randompattern of nanostructures configured according to a stacked optical datastorage format (the bottom image is the corresponding Fourier response).In particular, the stacked quasi-random pattern of nanostructuresincludes the quasi-random pattern of nanostructures shown in theleft-most image of FIG. 11F overlaid three times, in three differentorientations.

The top image of FIG. 11I also illustrates a stacked quasi-randompattern of nanostructures configured according to a stacked optical datastorage format (the bottom image is the corresponding Fourier response).In particular, the stacked quasi-random pattern of nanostructuresincludes the quasi-random pattern of nanostructures shown in the secondfrom the left image of FIG. 11H overlaid five times, in five differentorientations.

The number of optical data storage formats and the type of optical datastorage formats to be stacked may be selected to provide a particularFourier response for the stacked quasi-random pattern of nanostructures.By way of illustration, the stacked quasi-random pattern ofnanostructures may be configured to provide a ring in Fourier spacehaving a size which corresponds to a desired range of photon energiesand/or to provide a desired placement of the diffraction peaks. Thestacked quasi-random pattern of nanostructures may be configured toprovide a Fourier response which is characterized by a distribution ofk-values which is substantially within the k-values required to couplelight having a selected range of wavelengths (e.g., from about 315 nm toabout 2.5 mm, from about 315 nm to about 775 nm, etc.) to surfaceplasmons existing at the interface between the patterned layer and anoverlying layer. (See the right image in FIG. 1C for an optimaldistribution of k-values which is substantially within the k-valuesrequired to couple light having a range of wavelengths covering theentire solar spectrum.)

By using multiple parametric equations, such as the ones shown in FIG.12, to dictate the rotation speed (t) vs. the radial position (r1 or r2)of the raster head on which the laser is mounted, it is possible towrite overlapping data onto an optical media disc. Thus, the stackedoptical data storage formats described above and those illustrated inFIGS. 11G and 11I may be obtained using the same raster scan technologycurrently used to manufacture optical media discs in the industry.

The stamps used in the present methods comprise a stamping surface,which is the surface of the stamp to be pressed onto the surface of thelayer of the photonic device to be patterned. Thus, the stamping surfacedefines a negative replica of the quasi-random pattern of nanostructuresto be defined in the surface of the layer to be patterned. The stampsare derived from the pre-written optical media discs themselves. Asillustrated in FIG. 2A and further described in Example 1, below, astamp may be made by using the pre-written optical media disc as atemplate. In particular, a material may be cast onto an exposed surfaceof the recording layer from a section of the pre-written optical mediadisc and subsequently lifted off. Prior to lift off, the cast materialmay be cured for a period of time at an elevated temperature sufficientto harden the cast material. A variety of materials may be used for thestamp, e.g., a polymeric material such as polydimethylsiloxane (PDMS).

Alternatively, the stamp may be a material layer of the pre-writtenoptical media disc. As illustrated in FIG. 4C, a cross-sectional view ofa schematic of a pre-written optical media disc characterized by theBlu-ray optical data storage format is shown. The layer below therecording layer (i.e., the cover and hard coat layers) may be used asthe stamp.

In both embodiments, the stamp comprises a stamping surface whichdefines a negative replica of the quasi-random pattern of nanostructuresdefined in the recording layer of the pre-written optical media disc.This quasi-random pattern of nanostructures defined in the recordinglayer of the pre-written optical media disc may be configured accordingto any of the standard, non-standard, or stacked optical data storageformats described above.

The stamping surface may be modified prior to use in patterning thelayer of the photonic device. By way of illustration, the pits on thestamping surface may be etched so that they are sufficiently deep toaccommodate a desired thickness for the layer to be patterned.

The layer to be patterned by the present methods is a layer of aphotonic device. By photonic device, it is meant a device comprising aphotoactive layer capable of generating, manipulating or detecting lightwithin the device. Illustrative photonic devices include photovoltaiccells, photonic couplers, etc. However, the term “photonic device” asused herein excludes optical media discs.

The composition of the layer to be patterned depends upon the particularphotonic device and the particular portion of the photonic device toinclude the pattern. In some embodiments, the photonic device is aphotovoltaic cell. The photovoltaic cell may comprise a front electrodelayer, a back electrode layer, a photoactive layer between the front andback electrode layers, and an electron transport layer between the frontand back electrode layers. Any of these layers may be patterned by thepresent methods. Photoactive layers composed of a variety of materialsmay be used, e.g., organic materials such as PTB7:PC₇₀BM, P3HT:PCBM,etc.; or inorganic materials such as silicon, perovskites, group III-Vsemiconductors, group II-VI semiconductors, etc. Electrode layerscomposed of a variety of materials may be used, e.g., Al, Ag, Au, etc.The present methods may comprise additional steps related to forming theadditional layers over the patterned layer in order to complete thephotonic device.

As described above, a layer of a photonic device may be patterned with astacked quasi-random pattern of nanostructures which comprises aquasi-random pattern of nanostructures overlaid with one or moreadditional quasi-random pattern of nanostructures. Such stackedquasi-random patterns of nanostructures may be formed from a stampderived from a pre-written optical media disc characterized by a stackedoptical data storage format. In such an embodiment, the recording layerof the pre-written optical media disc defines a stacked quasi-randompattern of nanostructures configured according to the stacked opticaldata storage format and the stamping surface of the stamp defines anegative replica of the stacked quasi-random pattern of nanostructures.

Alternatively, a layer of a photonic device may be patterned with astacked quasi-random pattern of nanostructures by pressing a singlestamp derived from a pre-written optical media disc characterized by an“unstacked” optical data storage format (e.g., one of the standard ornon-standard optical data storage formats described above) multipletimes onto a surface of the layer. As another alternative, multiplestamps, each stamp derived from a pre-written optical media disccharacterized by a different unstacked optical data storage format, maybe individually pressed onto the surface of the layer. In theseembodiments, the type of stamps, the number of presses, the relativeorientation of the presses, and/or the number of stamps may be selectedto provide a desired stacked quasi-random pattern of nanostructures.

By way of illustration, a layer of a photonic device may be patternedwith the stacked quasi-random pattern of nanostructures shown in FIG.11E (top) by: first, pressing a stamp derived from a pre-written opticalmedia disc template characterized by the Blu-ray optical data storageformat in a first orientation onto the surface of the layer (see theleft image of FIG. 11D); second, pressing the stamp onto the surface ofthe layer in a second orientation (e.g., 60°) relative to the firstorientation (see the middle image of FIG. 11D); and third, pressing thestamp onto the surface of the layer in a third orientation (e.g., 120°)relative to the first orientation (see the right image of FIG. 11D). Thestacked quasi-random pattern of nanostructures shown in FIG. 11G (top)may be similarly made using a stamp derived from a pre-written opticalmedia disc template characterized by the optical data storage formatshown in the left-most image of FIG. 11F and pressing the stamp threetimes in three different orientations. The stacked quasi-random patternof nanostructures shown in FIG. 11I (top) may be similarly made using astamp derived from a pre-written optical media disc templatecharacterized by the optical data storage format shown in the secondfrom left image of FIG. 11H and pressing the stamp five times in fivedifferent orientations.

As another illustration, a layer of a photonic device may be patternedwith a stacked quasi-random pattern of nanostructures by: first,pressing a first stamp derived from a first pre-written optical mediadisc template characterized by a first non-standard optical data storageformat onto the surface of the layer; and second, pressing a secondstamp derived from a second pre-written optical media disc templatecharacterized by a second non-standard optical data storage format ontothe surface of the layer. Additional stamps, e.g., a third stamp, afourth stamp, etc. may be used.

The present methods of patterning layers of photonic devices may beextended to photolithography employing masks derived from pre-writtenoptical media discs. In a basic embodiment, a method of patterning alayer of a photonic device comprises aligning a mask on a surface of aphotoresist-coated layer of a photonic device, the mask comprising amasking surface which defines a negative replica of a quasi-randompattern of nanostructures defined in a recording layer of a pre-writtenoptical media disc. The mask is then illuminated with light underconditions sufficient to expose the photoresist-coated layer accordingto the quasi-random pattern of nanostructures defined in the recordinglayer of the pre-written optical media disc. Following standardphotolithographic techniques, the illuminated photoresist-coated layeris developed to remove portions of the photoresist coating to provideuncoated portions of the layer; the uncoated portions are etched; andany remaining photoresist coating is removed. By using this method, thesurface of the layer of the photonic device has a quasi-random patternof nanostructures defined therein which substantially matches thequasi-random pattern of nanostructures defined in the recording layer ofthe pre-written optical media disc.

In this embodiment, the description of the patterning method employingphotolithographic masks follows that described above with respect to thepatterning method employing stamps, with the exception of the method ofmaking the masks. Masks may be made similarly to the casting techniqueused to form the stamps, except that the material(s) cast (or deposited,grown, etc.) onto the pre-written optical media disc templates are thoseappropriate for carrying out photolithography, e.g., a layer of metalunderlying a layer of a transparent material.

In another embodiment, the “spikes” on the photolithographic masks madeas described above may be used to pick up and remove photoresist from aphotoresist-coated layer of a photonic device. This step may be repeatedmultiple times with rotation, leaving photoresist only in places wherethe pattern was not there (i.e., the white portions in the patterns ofFIGS. 11E, 11G, 11I). A typical photolithography process could then befollowed to provide the patterned layer.

The photolithographic masks and the photonic devices comprising thepatterned layer(s) made using the photolithographic masks are alsoprovided.

EXAMPLES Example 1: Repurposing Blu-Ray Movie Discs as Low-Cost,Quasi-Random Nanoimprinting Templates for Photon Management

Introduction

This example reports the use of pre-written Blu-ray movie discs asnanoimprinting molds for introducing quasi-random nanostructures intoorganic solar cells to enhance their efficiencies. Blu-ray discs aremass-produced data storage media with very low costs. Regardless of thecontent, the audio and video compression algorithms convert the datainto a high-entropy binary sequence before error-control coding andmodulation, eventually yielding a quasi-random arrangement ofsubwavelength “pits” and “lands” on the disc. This pattern issurprisingly well suited for photon management over the solar spectrum.In this example, a Blu-ray pattern is successfully imprinted onto theactive layer—and subsequently to the metal electrode—of polymer solarcells, leading to higher absorption and power conversion efficiencies.Additionally, the use of this technique for enhancing light trapping forother photoactive materials is demonstrated.

Methods

Fourier Transforms:

The two-dimensional discrete Fourier transform (DFT) of the images inFIGS. 1A-D (left images) were computed and the color-map customized toimprove the contrast of the FT images so as to better compare thefeatures.

Numerical Calculations:

The absorption was calculated by rigorous coupled wave analysis (RCWA),which is one of the most commonly used techniques to solve thescattering problem in Fourier space. For the multilayered dielectricstacks, Fourier expansions of both the field and the permittivity leadto an algebraic eigenvalue system for each layer. As quasi-randomnessalong one direction is of interest, a 1D simulation was implemented withperiodicity 10 μm. The number of Fourier components considered was 289and the convergence test was performed on the selection of thediffraction order to ensure the numerical accuracy.

Nanopatterned Mold Fabrication:

Polydimethylsiloxane (PDMS) nanopatterned molds were fabricated using adelaminated Blu-ray disc as the master. The Blu-ray movie disc(Supercop) was first trimmed around the edges using scissors, and thencarefully peeled off the cover layer (see FIG. 2A) to reveal the patternshown in FIG. 4A. No cleaning or secondary processing of the Blu-raydisc was required, as the cover layer effectively protects thenanopatterned surface after manufacturing. PDMS was cast on top of thedelaminated Blu-ray disc, and then cured overnight at 60° C. Afterpreparation, the PDMS mold could be lifted off the Blu-ray disc andreused several times. The cover layer, which was removed as shown inFIG. 2A, contains the same pattern as the PDMS mold and can also be usedas a nanoimprinting stamp. However, PDMS stamps may more reliablytransfer the 2D pattern to polymer thin films due to their flexibility,ductility, and favorable surface energy properties.

Device Fabrication:

FIG. 2A schematically shows the device fabrication procedure. Alldevices were fabricated on patterned ITO-coated glass substrates (20Ωsq⁻¹) that were pre-cleaned and treated with oxygen plasma immediatelybefore use. A sol-gel synthesis was employed for the ZnO electrontransport layer (ETL) following established procedures.³⁰ A solution of0.25M zinc acetate dihydrate and 0.25M ethanolamine in 2-methoxyethanolwas spin-coated through a PVDF filter and annealed at 180° C. for 20minutes to yield a 15 nm thick film. The 15 nm thickness of the ZnOfilms was verified using spectroscopic ellipsometry (J. A. WoollamM2000U). The active material was prepared by blendingpoly-[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl]-[3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7; 1-material) with (6,6)-phenylC₇₁-butyric acid methyl ester (PC₇₁BM; nano-C) at 1.0:1.5 in a solutionof 97 vol % chlorobezene (Sigma) and 3 vol % diodooctane (Sigma) andmixed for two days at 65° C. A solution concentration of 10 mg mL⁻¹ bytotal weight yielded an active layer thickness of 50 nm. Afterspin-coating, a PDMS stamp was placed on top of the active layer (forstamped devices) and all devices were placed under vacuum for 2 hours inorder to allow for evaporation of the diodooctane, and for PTB7 andPC₇₁BM domains to phase segregate. If the stamp was removed immediately,the nanopattern would fade away within a few hours, which was attributedto the residual solvent allowing the active material to reconfigure intoa lower energy (i.e. flatter) morphology. After carefully removing thestamp, all devices were moved to a thermal evaporator (Angstrom CovapII), where 10 nm of MoO₃ (Alfa Aesar, Puratronic, 99.9995%) and 100 nmof Ag were deposited. Control experiments were conducted with devicesstamped with a flat PDMS mold. The physical morphologies of theflat-stamped control and non-stamped active layers are comparable (SeeFIG. 10A), with the flat-stamped device being slightly smoother.

Characterization of Photovoltaic Cells:

Microscopic and morphological characterization was performed using anSEM (FEI Nova 600) or AFM (Park Systems XE-100), respectively.Reflection data was obtained with an Andor SR-303i_A spectrometercombined with Leica DMI 3000M microscope (10× objective, numericalaperture NA=0.3, 100 W halogen light source). Current density-voltagecharacteristics of all devices were measured under AM1.5 G illuminationusing an Oriel Xe solar simulator, employing filters to cut off gratingovertones. Corrected current density measurements were calculated aftermeasuring external quantum efficiencies under short circuit conditions(Enli Technology QE-R3018). A calibrated monosilicon diode with knownspectral response served as a reference. Averages over the best 12devices measured were used to obtain the device statistics.

Results and Discussion

Assessing the Blu-Ray Pattern for Light Trapping:

The arrangement of nanostructures within a light-trapping layerdramatically affects how photon energy is coupled into the plasmonic andwaveguide modes confined in the active layer of a solar cell²⁰. FIGS.1A-1D show the Fourier transforms of subwavelength features arranged inperiodic, random, quasi-random, and Blu-ray patterns. The red (inner)and blue (outer) circles in Fourier space mark the k-values required tocouple incoming light into the waveguide modes at the red- and blue-endsof the solar spectrum, respectively. Periodic (FIG. 1A) and random (FIG.1B) patterns yield wavevectors that are too discretized and too diffuse,respectively. On the other hand, quasi-random patterns (FIG. 1C) can beoptimized to yield Fourier spectra that are efficient at light trapping,but these patterns are typically prohibitively expensive to manufacture.In comparison, although still containing a periodic component, thepattern on a pre-written Blu-ray movie disc (FIG. 1D) produces aclose-to-optimized distribution of k-values.

The Blu-ray Disc (BD) standard²¹ was developed for high-density opticaldata storage, and has proven popular for distributing high-definitionmovies. The BD standard specifies that the track pitch is 320 nm and thepits are 130 nm wide and a minimum of 150 nm long (see a representativeatomic force microscopy (AFM) image in FIG. 4A). When writing to a disc,video signals are first compressed (e.g. MPEG4 format), resulting in abinary sequence with extremely high entropy per bit that is virtuallyindistinguishable from a random, uncorrelated sequence of bits²². Whenreading data from the disc, however, very short runs of successive zerosor ones yield low signal amplitudes, and very long runs are difficult todistinguish from noise due to scratches or fingerprints²¹. A modulationcode (MC) is therefore applied to the data, preventing very short (i.e.high-frequency) and very long (i.e. low-frequency) runs of zeros orones. Effectively, the MC-encoder transforms a random binary sequenceinto a quasi-random sequence with a tailored Fourier spectrum. As aresult of the data compression and MC-encoder algorithms, data writtento the disc results in a quasi-random, subwavelength pattern that isamazingly well suited for photon management over the solar spectrum,regardless of the movie content or area selected (See FIG. 6).

Simulated Active Layer Absorption:

The light trapping effect of the Blu-ray structure on a typical polymersolar cell with a PTB7:PC₇₁BM active layer²³ (see FIG. 7A for the devicestructure) was numerically assessed by performing one-dimensionalrigorous coupled-wave analysis (RCWA)²⁴. The absorption was modeled bothparallel and perpendicular to the track, as marked by the four lines onthe AFM image shown in FIG. 7B. The corresponding absorption spectrafrom 315 nm to 775 nm was calculated both under transverse-electric(FIG. 7C) and transverse-magnetic (FIG. 7D) polarizations as referencedto the plane of the cross section. When compared to a non-patternedsolar cell, the spectra of Blu-ray patterned devices display broadbandenhancement under both polarization conditions^(16,25,26). Extremelyhigh enhancement 113.9% can be found in the region between 700 nm to 775nm for TM polarization, indicating the underlying importance of thelight trapping effect in the weak absorption region of the active layer.The overall broadband absorption enhancement of a Blu-ray patterneddevice of 18.2% was calculated by averaging over the simulation results,as shown in FIG. 7E.

Device Fabrication and Characterization:

FIG. 2A illustrates the typical procedure for fabricating Blu-raypatterned polymer solar cells. The Blu-ray disc was first delaminated toexpose the pattern of pits and lands, which was replicated on apolydimethylsiloxane (PDMS) stamp. The AFM image of the resulting stampin FIG. 2B confirms that it is a high quality negative replica of theBlu-ray pattern with features as small as 150 nm across and 25 nm high.Next, the pattern was imprinted into a pre-fabricated polymer activelayer by contact molding using the stamp, followed by electrodedeposition to complete the device. The final device structure isillustrated in FIG. 2C.

The AFM images in FIG. 3A clearly show successful transfer of theBlu-ray pattern to the active layer after nanoimprinting, in starkcontrast with a non-patterned active layer. Line scans (FIG. 3A, bottom)show that the feature sizes are highly consistent with those of both thestamp and the original Blu-ray disc. The transferred pattern (c.a. 1cm²) displayed uniform iridescent reflection (FIG. 3B), demonstratingthe reliability of this nanoimprinting process over large areas. Thereflection (R) spectra of both non-patterned and Blu-ray patterned solarcells were measured and plotted 1-R, i.e. absorption in FIG. 3C. Theabsorption of the Blu-ray imprinted cell is significantly enhanced by21.8% over the entire absorption profile. Notably, the benefit of lighttrapping is most pronounced after 700 nm, reaching 49.0%, where thematerial absorbs weakly. Both observations are consistent with thesimulation shown in FIGS. 7A-E. The enhanced broadband absorption ofBlu-ray patterned solar cells indeed led to improved external quantumefficiencies (EQE) (FIG. 3D). The overall EQE enhancement averaged overthe entire absorption profile is 30.8%, while the averaged enhancementis 85.0% for wavelengths greater than 700 nm. As a result, the patternedcells delivered 16.9% higher short-circuit current densities (J_(sc)),eventually leading to a power conversion efficiency enhancement of 11.9%(See FIG. 8 and Table 1, below, for detailed performance results).Although higher J_(sc) values have been reported for PTB7:PC₇₁BM solarcells with thicker active layers (˜100 nm)²³, in the current example, anactive layer thickness of 50 nm was chosen to best demonstrate the lighttrapping effect while avoiding shorting caused by imprinting with the 25nm deep pattern. However, the depth of the pits could be modified duringthe mold fabrication step (e.g. via etching) to accommodate the needs ofalternative thicknesses for device optimization.

TABLE 1 Solar cell performance. J_(sc), V_(oc), fill factor (FF),efficiency and corresponding enhancement factor of the all devices,including control devices stamped with the flat mold. The error valuessignify one standard deviation. J_(sc) Efficiency (mA cm⁻²) V_(oc) (V)FF (%) Sample Blu-ray  −7.16 ± 0.48 0.720 ± 68.2 ± 3.51 ± patterned0.012 2.14 0.33 Flat-  −5.92 ± 0.15 0.711 ± 68.1 ± 2.87 ± “patterned”0.023 1.29 0.34 Non-patterned  −6.13 ± 0.32 0.728 ± 70.4 ± 3.13 ±Enhancement 0.012 2.45 0.30 (%) Blu-ray/Flat   20.88 ± 1.49  1.3 ± 0.1 ±0.00 22.4 ± 0.05 3.34 BluRay/Non-   16.85 ± 1.43  −1.0 ±   −3.2 ± 11.9 ±stamped 0.02 0.15 1.58

This example demonstrates the use of pre-written, Blu-ray movie discs asextremely low-cost nanoimprinting templates for creating photonmanagement nanostructures in thin film solar cells. Mode analysis (FIGS.9A-9C) demonstrates that the Blu-ray patterns can be broadly applied forlight trapping in other types of solar cells, including those made withamorphous Si, III-V and II-VI semiconductors, and perovskite compounds.In contrast to previous attempts to make use of the periodicmicro-/nanostructures in blank optical storage media²⁷⁻²⁹, this examplemakes use of the quasi-random patterns in information-laden discs anddemonstrates that the repurposing of a low cost consumer product leadsto much higher-end, value-added applications.

Example 2: Optical Disc Nanopattern Manipulation

Introduction

Surface nanopatterns, such as periodic, random, and quasi-randompatterns, are useful for photon management applications such aswaveguides and light trapping. However, periodic patterns that couple todiscrete peaks in Fourier space, and random patterns that scatter over alarge range of Fourier space both have limited applicability because oftheir limited tunability. By contrast, quasi-random (QR) patterns can bedesigned to have feature sizes that couple to specific photon energyranges, while still displaying a high degree of randomness. However,such patterns are often expensive to produce.

As described in Example 1, the nanopatterns found within Blu-raydiscs—and indeed all pre-written optical discs—are QR in nature alongthe read direction of the disc while being quasi-periodic in the radialdirection. A simplified description of an algorithm for encoding mediafiles onto optical discs (e.g. MPEG-4) is shown in FIG. 11A. In a firststep, compression coding is applied to uncompressed media to provide asubstantially random binary sequence. In a second step, a standard errorcontrol coding is applied to the substantially random binary sequence totransform it into a quasi-random binary sequence having a particularFourier spectrum. The QR nature of optical disc nanopatterns resultsfrom the error-control portion of the algorithm. Error controllingprevents patterns from being written to the disc that may cause readerrors, such as a very long string of consecutive bits or strings ofrapidly alternating bits. This error control coding ensures the lengthof pits and lands falls within a specified size range, thus creating aQR sequence. FIG. 11B shows the pattern of a recording layer of ablank/rewritable optical disc (top) and its corresponding Fourierspectrum (bottom). FIG. 11C shows the pattern of a recording layer of apre-written optical disc (top) and its corresponding Fourier spectrum.When compared to blank optical discs, pre-written optical discs haverich Fourier spectra in the x-direction. In Example 1, it was shown thatthe Fourier response of pre-written Blu-ray discs is well matched forcoupling light into the waveguide modes in a prototypical solar cell,increasing the device efficiency. However, the industry standard errorcontrol coding algorithms create the same Fourier responses from everydisc, regardless of the original data written.

This example demonstrates manipulation of optical disc patterns byoverlaying multiple patterns (FIGS. 11D-11E), tuning the allowednanofeature size ranges (FIGS. 11F-11G), and varying the track spacing(FIGS. 11H-11I). The resulting polarization-independent, highly tunableFourier transforms may be applied to myriad photon managementapplications, while still maintaining many of the advantages of theoriginal mass-produced optical disc formats.

Methods

Simulations were carried out in Matlab. The simulations were based onthe feature sizes dictated by the Blu-ray Disc standard—each bit,corresponding to part of a pit or land, was assumed to be 75 nm long and130 nm wide with a track pitch of 320 nm—and scaled linearly toapproximate the conversion to DVD and CD standards. It should be notedthat the minimum number of contiguous bits to make a pit or land is two,so the minimum length of either a pit or land is 150 nm. In thisexample, it was assumed the length (L) distribution of pits and landsfollowed a l/L distribution with the minimum feature length equal to 2and the maximum equal to 7. Fourier transforms of these images werecalculated, as shown in FIGS. 11B, 11C, 11E, 11G, and 11I, using the FFTfunction. On rewritable Blu-ray discs, the physical structure of thedisc doesn't change upon writing, with the information instead beingencoded in a phase-transformation or dye material, so they weresimulated as alternating rows of pits and lands. Images were overlaidusing Boolean algebra and then cropped to remove areas of only partialoverlay.

Results and Discussion

The nanopatterns found within pre-written optical discs were simulatedas two-dimensional binary images and their corresponding Fouriertransforms were computed, as detailed in the Methods section above andshown in FIG. 11C. One limitation of the optical disc pattern of FIG.11C is its polarization dependence, since it is only QR in the readdirection. In order to reduce the polarization-dependence, threesimulated images were overlaid, with two of the images rotated at 60°and 120° with respect with the first (original) image, using binaryaddition to achieve the image shown in FIG. 11E (top). The resultingFourier transform in FIG. 11E (bottom) is a rotationally symmetric ringthat is nearly polarization-independent for incoming light, with someremnant periodicity seen from the first order peaks at multiples of 60°.More images may be overlaid to increase the rotational symmetry.

Further control over the FT is exerted by controlling the data writtento the disc. By restricting the size of the nanofeatures to a narrowdistribution, the Fourier response is limited to a narrow band. Whenthese patterns are overlaid, a narrow ring in Fourier space is observed,which corresponds to a narrow band of photon energies being manipulated.Such rings are frequently observed in X-ray spectra (i.e. the FT) ofcreatures that exhibit structural coloration, such as brightly coloredbird feathers. The size of this ring—and thus the manipulated photonenergies—can be tuned by tuning the distribution of the feature sizes ofthe nanopattern. By way of illustration, the real-space images (top row)and Fourier space images (bottom row) of FIG. 11F demonstrate the effectof the contiguous length of pits and lands on the Fourier transform.They show (left-most) 2 or 3 contiguous bits (next adjacent) 4 or 5contiguous bits, (next adjacent) 6 or 7 contiguous bits, and(right-most) 8, 9, . . . 20 contiguous bits forming pits and lands.Smaller features (i.e. fewer contiguous bits) create patterns in Fourierspace that are further from the central vertical line along which thefirst and second order diffraction peaks fall. The pattern in theleft-most image was overlaid three times to create the image shown inFIG. 11G (top). The ring shown in FIG. 11G (bottom) was optimized sothat the ring overlaid with the first order diffraction peak thatresults from the track periodicity.

By further modulating the track periodicity, it is possible to shift theplacement of the first order peaks. By using quasi-random spacingbetween tracks, the periodicity due to the tracks can be eliminated,instead resulting in a blurred out vertical line. By completelyeliminating the track spacing, the periodicity of the patterns can befurther reduced. By way of illustration, the real-space images (top row)and Fourier space images (bottom row) of FIG. 11H demonstrate the effectof track spacing on the diffraction peaks. Each of the images uses 6-7contiguous bits, as shown in FIG. 11F (second from the left image). Theyshow that a track spacing dictated by a typical optical disc standardyields primary, secondary, and higher order diffraction peaks as shownin the Fourier transform, which are caused by the track spacing(left-most image). By increasing the track spacing by a factor of 3, thediffraction peaks move closer to the center of the Fourier space, asshown in the next adjacent image. This change in diffraction peaklocation is beneficial when images are overlaid, as the diffractionpeaks can be made to lie on the circle. It should be clear to see thatchanging the track spacing to be the same size as the median featuresize (e.g. the length of the 6-7 bit features in this case) causes thefirst order diffraction peaks to overlay on the circle. This pattern wasoverlaid 5 times in order to make the pattern shown in FIG. 11I. Asshown in the next adjacent image in FIG. 11H, by assigning a randomtrack spacing (in this case the vertical spacing between pits and landsis allowed to range between 0 and 3 times the typical spacing), thediffraction peaks become undefined and smeared out along the verticalcentral line. Finally, as shown in the right-most image, by setting thetrack spacing to zero, the diffraction peaks due to the track disappearand only the diffraction peaks due to the pit and land width is evidentin the Fourier transform.

The industry-standard error-control algorithms used to write opticaldiscs create the same QR Fourier response regardless of disc content.The optical disc nanopattern manipulation techniques described in thisExample liberate optical disc formats for a wider range of photonmanagement applications beyond their intended use. Indeed, thesetechniques allow for similar tunability (e.g. the shape of the Fourierresponse) afforded by other deterministic methods of creating QRpatterns, such as spinodal decomposition. However, other QR patterngeneration techniques typically require low-throughput fabricationtechniques such as electron beam lithography and require either tiledimages or very large file inputs. Further, the techniques described inthis Example allow for enhanced control for coupling to many materialcombinations with a wide range of optical properties, and over much ofthe solar spectrum. Finally, the techniques described in this Example,particularly the data-manipulation and track-modulation modifications,are compatible with current optical disc production techniques. Currentindustrial capabilities for optical disc manufacturing are enormous, yetmedia content is increasingly distributed via online downloads andstreaming rather than via optical discs. Example 1 involved repurposingoptical discs for a myriad of value-added applications related to photonmanagement. This Example involves repurposing the manufacturingequipment used to make optical discs for such value-added applications.

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The word “illustrative” is used herein to mean serving as an example,instance, or illustration. Any aspect or design described herein as“illustrative” is not necessarily to be construed as preferred oradvantageous over other aspects or designs. Further, for the purposes ofthis disclosure and unless otherwise specified, “a” or “an” means “oneor more”.

The foregoing description of illustrative embodiments of the inventionhas been presented for purposes of illustration and of description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed, and modifications and variations are possible inlight of the above teachings or may be acquired from practice of theinvention. The embodiments were chosen and described in order to explainthe principles of the invention and as practical applications of theinvention to enable one skilled in the art to utilize the invention invarious embodiments and with various modifications as suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto and theirequivalents.

What is claimed is:
 1. A photonic device comprising a photoactive layerand at least one additional layer, wherein a surface of the photoactivelayer or a surface of the at least one additional layer has imprintedthereon a quasi-random pattern of nanostructures corresponding to aquasi-random pattern of nanostructures defined in a recording layer ofan optical media disc, wherein the quasi-random pattern ofnanostructures defined in the recording layer of the optical media discand the imprinted surface comprises a plurality of pits and landsarranged in a plurality of tracks, each track comprising both pits andlands, the tracks aligned along their longitudinal axes.
 2. The photonicdevice of claim 1, wherein the quasi-random pattern of nanostructuresdefined in the recording layer of the pre-written optical media disc andthe imprinted surface is configured according to an optical data storageformat, the optical data storage format characterized by one, two, orall three of: a pit width of about 130 nm, a minimum pit length of about150 nm, and a track pitch of about 320 nm.
 3. The photonic device ofclaim 2, wherein the optical data storage format is characterized by thepit width of about 130 nm, the minimum pit length of about 150 nm, andthe track pitch of about 320 nm.
 4. The photonic device of claim 1,wherein the quasi-random pattern of nanostructures defined in therecording layer of the optical media disc and the imprinted surface is astacked quasi-random pattern of nanostructures comprising thequasi-random pattern of nanostructures comprising the plurality of pitsand lands arranged in the plurality of tracks, the tracks aligned alongtheir longitudinal axes, which is configured according to a firstoptical data storage format, and at least one additional overlyingquasi-random pattern of nanostructures comprising a plurality of pitsand lands arranged in a plurality of tracks, the tracks aligned alongtheir longitudinal axes, which is configured according to a secondoptical data storage format.
 5. The photonic device of claim 4, whereinan angle θ defined by the longitudinal axes of the tracks of the atleast one additional overlying quasi-random pattern of nanostructuresrelative to the longitudinal axes of the tracks of the quasi-randompattern of nanostructures is in the range of 0°<θ<360°.
 6. The photonicdevice of claim 5, wherein the quasi-random pattern of nanostructuresand the at least one additional overlying quasi-random pattern ofnanostructures are configured according to the same optical data storageformat.
 7. The photonic device of claim 6, wherein the same optical datastorage format is an optical data storage format characterized by one,two, or all three of: a pit width of about 130 nm, a minimum pit lengthof about 150 nm, and a track pitch of about 320 nm.
 8. The photonicdevice of claim 7, wherein the same optical data storage format ischaracterized by the pit width of about 130 nm, the minimum pit lengthof about 150 nm, and the track pitch of about 320 nm.
 9. A photovoltaiccell comprising a front electrode layer, a back electrode layer, and aphotoactive layer between the front and back electrode layers, wherein asurface of a layer of the photovoltaic cell has imprinted thereon aquasi-random pattern of nanostructures corresponding to a quasi-randompattern of nanostructures defined in a recording layer of an opticalmedia disc.
 10. The photonic device of claim 9, wherein the imprintedsurface is the surface of the photoactive layer.
 11. The photonic deviceof claim 9, further comprising an electron transport layer between thefront and back electrode layers.
 12. The photonic device of claim 1,wherein the quasi-random pattern of nanostructures defined in therecording layer of the optical media disc and the imprinted surface isconfigured to provide a Fourier response characterized by a distributionof k-values which couples sunlight having wavelengths in the range offrom about 315 nm to about 2.5 μm to surface plasmons at an interfacebetween the imprinted surface and an overlying layer of the photonicdevice.
 13. The photonic device of claim 12, wherein the distribution ofk-values couples sunlight having wavelengths in the range of from about315 nm to about 775 nm.